Increasing environmental awareness relating to global climate change and the desire to reduce dependence on imported fuel sources have helped drive efforts to develop technologies that enable more efficient usage of electrical energy. In the United States, the amount of electricity consumed for lighting is significant, accounting for around 18-20% of the total energy usage in buildings. While the push to adopt compact fluorescent lighting (CFL) has achieved some progress toward the goals of energy efficiency, there are several drawbacks to the CFL bulbs, which include slow starting, inability to dim, limited directionality, and their use of small amounts of mercury, which in some states requires discarded bulbs to be treated as hazardous waste.
The U.S. Congress passed the Energy Policy Act of 2005 directing the U.S. Department of Energy to carry out a “next generation lighting initiative” to promote the development of high-efficiency advanced solid state lighting technologies that could replace conventional lighting sources, including incandescent, halogen, fluorescent and gas-discharge lighting. Solid state lighting technologies include light emitting diodes (LEDs) and lasers. To be accepted as replacements for conventional lighting applications, it is essential that the solid state lighting devices emit light that falls within the standard range of “white light”, e.g., warm, neutral and cool-white with typical Correlated Color Temperatures (CCTs) that range from 2670K to 3500K for warm-white, 3500K to 4500K for neutral-white, and 4500K to 10000K for cool-white. While LEDs have long been used as indicator lights, typically colored, in a wide variety of industrial, commercial and consumer electronic devices, it was not until relatively recently that the technology had advanced to the point that white light emitting LEDs were capable of generating sufficiently intense light to be used in traffic lights, outdoor lighting, particularly solar lighting, vehicle tail lights and flashlights, among other applications. These newer generation LEDs have been received with much enthusiasm. For example, beginning in model year 2007, nearly every new car on the market incorporated LED tail lights.
It is estimated that white light emitting diodes (LEDs) are four times more efficient than incandescent and halogen sources and 30% more efficient that linear fluorescent sources. LED lighting has many advantages, including ultra long source life, low power consumption, low maintenance, no UV or IR radiation, cool beam of light, digitally controllable and sustainability.
A variety of white LEDs are currently available commercially, however they cannot yet compete with traditional light sources on the basis of performance or cost. True energy efficiency means using the most efficient light source or system that is capable of providing the amount and quality of light needed. Ongoing research and development efforts are directed to improving the performance of white LEDs to levels suitable for general lighting applications.
Most LEDs are formed by growing epitaxial layers, including p-type and n-type layers, on a growth substrate. A light-emitting active layer is sandwiched between the n- and p-layers. Green, blue, and ultraviolet LEDs are typically gallium-nitride based, where the growth substrate may be either sapphire, SiC, silicon, SiC-on-insulator (SiCOI), or other engineered substrate. Infrared, red, and amber LEDs are typically some combination of AlInGaPAs and grown on a GaAs or InP substrate. White LEDs have primarily been formed from aluminum or indium gallium nitride (Al/InGaN) on substrates of silicon carbide (SiC) or gallium nitride (GaN), however, zinc oxide has been the object of research for use in white LEDs. Most reported work to date on white LEDs has utilized thin film planar technology, although many researchers in the field are directing their attention to nanostructures such as nanowires, nanorods, nanofibers, etc. to exploit the greatly increased carrier injection surface areas that these structures provide.
Zinc oxide (ZnO) is a wide direct bandgap semiconductor (Eg=3.4 eV) that displays unique features that make it particularly attractive for use in solid state lighting and other applications. Some of these features include that it has large exciton binding energy Eb=60 meV), suggesting that ZnO-based lasers should have efficient optical emission and detection, large piezoelectric, and ferromagnetic coefficients with a predicted Curie temperature above room temperature when doped with transition metals. The cost and ease of manufacture of ZnO is attractive when compared against other common semiconductor materials. It has excellent radiation resistance (2 MeV at 1.2×1017 electrons/cm2), which is desirable for radiation hardened electronics. Zinc oxide has high thermal conductivity (0.54 W/cm*K). It has strong two-photon absorption with high damage thresholds, which is important for use in optical power limiting devices. Additional features of ZnO are detailed in U.S. Patent Publication No. 2005/0285119 A1 of Burgener, II et al., which is incorporated herein by reference.
The availability of a rich genre of nanostructures make ZnO an ideal material for nanoscale optoelectronics, electronics, and biotechnology. Functional devices such as vertical nanowire (NW) FETs, piezoelectric nano-generators, optically pumped nano-lasers, and biosensors have already been demonstrated. However, similar to other wide bandgap semiconductors, such as GaN, unintentionally doped ZnO is intrinsically n-type and obtaining p-type doping has proven extremely challenging. Although p-type conduction in ZnO thin film has been reported (see, e.g., D. F. Look et al., “The Future of ZnO Light Emitters”, Phys. Stat. Sol., 2004, incorporated herein by reference, which summarizes data on p-type ZnO samples reported in the literature), it still remains elusive, and no p-type ZnO NWs have been reported as yet. Complementary doping (both n-type and p-type doping) is essential for functional device applications and the lack of p-type ZnO NWs is currently a major factor precluding the realization of a wide range of functional nanodevices based on ZnO.